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Re: C/2024 G3
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After taking a another look at Michael's picture, I just realized from the star on the upper left that the image may have been sharpened with a high pass filter, which may be making the spine look sharper/narrower than it actually is. If the spine is actually a fairly broad/diffuse stripe, it may just be an effect of larger dust grains fragmenting into smaller dust grains, which are rapidly pushed tailward out of the coma by solar radiation pressure. That's not too unusual of a dust feature at this distance, and if that's what it is, this feature should soon fade/invert into a more classic "shadow of the nucleus" structure closer to the Sun. There, the fragmentation happens so rapidly/close to the nucleus that the outflowing gas still has a chance to drag along the newly small dust grains at a similarly high speed as the small grains ejected straight from the nucleus, thus puffing all the dust into the large coma. Qicheng
On Saturday, December 28, 2024 at 07:06:40 p.m. UTC, Qicheng Zhang via groups.io <qzalaska@...> wrote:
The comet's absolute magnitude is ~7, so it's technically brighter than the Bortle limit for all q. Of course, it's also dynamically old, but with a dynamical age on the order of only ~1 orbit, and disintegration isn't too abnormal for such comets, even when brighter than the Bortle limit. C/2021 A1 was a recent example that comes to mind. But many also do survive, like C/2002 V1. A narrow dust spine (at least before perihelion, as in the present case) means there's now considerable dust being released at very low velocity, which requires that dust be from a source far from the nucleus (as outflowing gas would push any dust near the nucleus itself at high velocity into the main coma). That suggests the comet started to disrupt, with fragments breaking off and drifting away from the nucleus before crumbling further into the low velocity, spine-forming dust. This sort of structure is likely also responsible for the triangular shape the gas coma of a disrupting comet often takes, but this comet may be too dusty for its gas coma to be seen. At the moment, the comet still has a large, primary coma, which indicates there's at least one large, active nucleus present. If the large coma fades out in favor of the spine (which is a distinct possibility, but far from guaranteed at this point), that would signal the end of this nucleus. Qicheng
On Saturday, December 28, 2024 at 12:29:58 p.m. UTC, Nick James <comets@...> wrote:
Hi all, I have been observing C/2024 G3 for the last few weeks using T70 at X07 in Chile. This instrument is able to get down to the mountain ridge horizon which is around 4 degrees at that azimuth and the sky transparecy at X07 is very good. T70 is a very short FL instrument (actually a Samyang 135mm FL lens) but it is good for magnitude estimates of bright comets although the astrometry I get from the images is a bit ropey. This morning (Dec 28.35) I had a total magnitude of 6.0 unfiltered using Comphot and Gaia DR3 G mags. Michael's estiamate at around the same time using T75 was 5.8 (taken from COBS). These are fitting the lightcurve well so there is no sign of any abnormal activity at the moment. Using astrometry from the MPC up to December 23 and my more recent astrometry with the 135mm lens there appears to be no obvious improvement to the residuals by including non-gravitational forces. This was using Findorb. My image from this morning is here: Other images and data are here: Here's hoping that this comet survives to provide a nice display post-perihelion. Sadly, us northern observers won't get much of a chance to see it so we will be relying on reports from the southern hemisphere. Nick James. BAA Comet Section. On 28/12/2024 07:42, Michael Mattiazzo via groups.io wrote: > Howdy Folks, > >? > > C/2024 G3 ATLAS has been kept under extensive observation by several observers, despite its poor location. > > Attached a light curve showing a steady brightening toward perihelion that could see it reach magnitude -3, if it survives. > > I¡¯ve also attached? images from Dec 25 and 26 showing a curious spine feature (often seen in outbursting comets) but this comet is not in outburst. > > Hopefully there is sufficient nuclear material to keep going. It is a very dusty comet as seen with the parabolic hood. > > This situation reminds me of C/2002 V1 NEAT in March 2023 > >? > > Cheers, > > Michael > > > > > > > |
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